This invention relates generally to the production of radiation, particularly extreme ultraviolet and soft x-rays, with a shaped, extended capillary electric discharge source for projection lithography.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85 (xe2x80x9cAngstromxe2x80x9d)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced because the mask does not have to be positioned within microns of the wafer as is the case for proximity printing. The cost of mask fabrication is considerably less because the features are larger. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of EUV radiation in bands at which multilayer coatings have been developed (i.e., xcex=13.4 nm, xcex=11.4 nm) allows the use of near-normal reflective optics. This in turn has lead to the development of lithography camera designs that are nearly diffraction limited over useable image fields. The resulting system is known as extreme UV (xe2x80x9cEUVLxe2x80x9d) lithography (a.k.a., soft x-ray projection lithography (xe2x80x9cSXPLxe2x80x9d)).
A favored form of EUVL projection optics is the ringfield camera. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow arcuate fields of aberration correction located at a fixed radius as measured from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of Jewell et al., U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width defines a region in which features to be printed are sharply imaged. Outside this region, increasing residual astigmatism, distortion, and Petzval curvature at radii greater or smaller than the design radius reduce the image quality to an unacceptable level. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image and use of object:image size reduction of, for example, 4:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
Sweatt al. al. U.S. Pat. No. 6,118,577 discloses a condenser system that couples radiation from a small diameter source to a ringfield camera. The condenser system typically includes six substantially equal radial segments of a parent aspheric mirror, each having one focus at the radiation source and line focus filling the object field of the camera at the radius of the ringfield and each producing a beam of radiation. The condenser system also includes a corresponding number of sets of correcting mirror means which are capable of translation or rotation, or both, such that all of the beams of radiation pass through the real entrance pupil of the camera and form a coincident arc image at the ringfield radius.
The overall layout of an EUV lithography system used with the Sweatt condenser is shown in FIG. 4. The radiation is collected from the source 22 by mirror segments 30 (referred to collectively as the xe2x80x9cC1xe2x80x9d mirrors) which create arc images that are in turn are rotated by roof mirror pairs illustrated collectively as mirrors 40 and 50 (referred to as the xe2x80x9cC2xe2x80x9d and xe2x80x9cC3xe2x80x9d mirrors, respectively). Beams of radiation reflected from mirrors 50 are reflected by a toric mirror 60 (or C4 mirror) to deliver six overlapped ringfield segments onto reflective mask 70. Mirror 31 creates an arc image and roof mirror pair 41 and 51 rotates the arc image to fit the slit image and translate it to the proper position. Similar arc images are created and processed by mirror combinations 32, 42, and 52, and so on. Mirrors 41, 42, and 43 are parts of different and unique channels; and the group of mirrors 44, 45, and 46 is a mirror image of the group of mirrors 41, 42, and 43, respectively. An illustrative arc 71 is shown on mask 70. The EUV lithography system further includes a ringfield camera 77 having a set of mirrors which images the mask using the radiation onto wafer 78.
Despite the advantages of the Sweatt condenser system, the art is still searching for improved efficiency. Achieving sufficient EUV flux at the wafer to support a high wafer throughput commercial EUV lithography xe2x80x9cstep-and-scanxe2x80x9d exposure tool is a significant challenge. Of the many elements that impact tool throughput, EUV source power and condenser efficiency both have tremendous leverage. For example, eliminating a single mirror in a condenser can increase flux at the wafer by a factor of (Rmirror)xe2x88x921 or approximately 1.5x.
The present invention is based in part on the recognition that employing a source of radiation, such as an electric discharge source, which is equipped with a capillary region that is configured into some predetermined shape, such as an arc or slit, can significantly improve EUV flux. One reason is that the condenser which delivers critical illumination to the reticle can be simplified from five or more reflective elements to a total of three or four reflective elements thereby increasing condenser efficiency. In this regard, preferably the dimensions of the non-circular shaped capillary bore correspond to that of the desired image that is focused by the camera. In the case where the inventive capillary discharge source is used in an EUV lithography system where the camera focuses arc or slit shaped images, the capillary discharge source has a bore having a length to width ratio that substantially matches that of the arc or slit shaped image that is focused by the camera. This enables the employment of a simpler condenser with fewer mirrors since the magnification parallel and perpendicular to the arc or slit can be approximately equal.
Accordingly, in one embodiment the invention is directed to a capillary discharge source that includes:
a body constructed from a dielectric material that defines a capillary with a bore having a non-circular shaped cross section; and
a gaseous species inserted into the capillary, wherein the capillary is used to generate radiation discharges.
In a preferred embodiment, the bore has a proximal end and a distal end and the source further includes:
(i) a source of gas that is in communication with the capillary bore;
(ii) a first electrode positioned at the distal end of the bore;
(iii) a second electrode at a reference potential positioned at the proximal end of the bore; and
(iv) a source of electric potential that is selectively connectable to the first electrode.
In another embodiment, the invention is directed to a source of radiation that includes:
means for generating radiation; and
a channel having a non-circular cross section that is coupled to the means for generating radiation so that radiation emanating from the source comprises a beam of radiation having a non-circular cross section.
In a further embodiment, the invention is directed to a photolithography system for projecting a mask image onto a wafer that comprises:
a ringfield camera;
a capillary discharge source that includes:
(i) a body constructed from a dielectric material that defines a capillary with a bore having a non-circular shaped cross section; and
(ii) a gaseous species inserted into the capillary, wherein the capillary is used to generate radiation discharges;
a condenser for processing the source radiation to produce a ringfield illumination field and for illuminating a mask;
a mask that is positioned at the ringfield camera""s object plane and from which the mask image in the form of an intensity profile is reflected into the entrance pupil of the ringfield camera; and
a wafer onto which the mask imaged is projected from the ringfield camera.
In yet another embodiment, the invention is directed to a photolithography system for projecting a mask image onto a wafer that includes:
a ringfield camera;
a source of radiation that includes:
(i) means for generating radiation; and
(ii) a channel having a non-circular cross section that is coupled to the means for generating radiation so that radiation emanating from the source comprises a beam of radiation having a non-circular cross section;
a condenser for processing the source radiation to produce a ringfield illumination field and for illuminating a mask;
a mask that is positioned at the ringfield camera""s object plane and from which the mask image in the form of an intensity profile is reflected into the entrance pupil of the ringfield camera; and
a wafer onto which the mask imaged is projected from the ringfield camera.
In an additional embodiment, the invention is directed to a method of producing radiation that includes the steps of:
(a) providing a capillary discharge plasma source that comprises a body that defines a capillary bore having a non-circular shaped cross section;
(b) introducing gaseous species into the capillary bore; and
(c) creating a plasma within the capillary bore thereby producing radiation of a selected wavelength that is emitted from the capillary bore whereby the emitted radiation has a non-circular shaped cross section which matches that of the capillary bore.